INFLUENCE OF NONINVASIVE VENTILATION BY BIPAP ON EXERCISE TOLERANCE AND RESPIRATORY MUSCLE STRENGTH IN CHRONIC OBSTRUCTIVE PULMONARY DISEASE PATIENTS (COPD)

O objetivo deste estudo foi avaliar o efeito do BiPAP, através de máscara nasal, na tolerância ao exercício físico e no desempenho muscular respiratório em pacientes com diagnóstico clínico e espirométrico de DPOC, moderado/grave (VEF1 < 60% do previsto). Com VEF1/CVF <70% do previsto e idade média de 59,4±8,9 anos, dez pacientes com doença pulmonar obstrutiva crônica (DPOC) foram tratados com 30 minutos de BiPAP (IPAP=10-15 e EPAP=4 cmH2O), em três sessões semanais, durante dois meses. Antes e após o tratamento mediu-se a espirometria, a força muscular inspiratória (PImax) e expiratória (PEmax) e a distância percorrida em seis minutos (TC6). Foram constatados aumentos significativos (Wilcoxon, p<0,05) na média da PImax (de -55±17 para -77±19 cmH2O), da PEmax (de 75±20 para 109±36 cmH2O) e da distância percorrida (de 349±67 para 448±75 metros). Com base nesses resultados conclui-se que o BiPAP melhorou o desempenho muscular respiratório e a tolerância ao exercício físico nesses pacientes com DPOC.

As COPD patients present ventilatory limitations that lead to progressive intolerance to efforts (6) , due to dyspnea, weakness and deconditioning of respiratory and peripheral muscles (7) , making them vulnerable to hospitalization, this study aims to assess the effects of bi-level NIV in COPD patients on exercise tolerance and respiratory muscle strength.Experimental procedure: Before and after treatment, patients were submitted to the following assessments:

MATERIALS AND METHODS
-Spirometry: carried out by means of a Vitalograph spirometer, model 2021, according to American Thoracic Society (8) standards, to characterize the degree of obstructive pulmonary disorder.
-Respiratory Muscle Strength: obtained by using a Ger-Ar mano-vacuummeter scaled in cmH 2 O.
Maximum inspiratory pressure (MIP) and maximum expiratory pressure (MEP) were measured according to earlier studies (9) , with the individual in the orthostatic position and with a nose clip.MIP was measured close to the residual volume after maximum expiration.MEP was measured close to total pulmonary capacity (TPC) after maximum inspiration.Individuals were oriented to sustain pressure for more than a second and each maneuver was realized at least three times.For the sake of analysis, the highest result was taken into account; -Six-Minute Walking Test (6MWT): to evaluate exercise tolerance, patients were submitted to a 6MWT in a flat level corridor of 30 meters length and 1.5 meters width, demarcated every 2 meters.
Patients were advised to take a light meal about two hours before the test and not to perform any intense physical exercise, nor take medication during the 24 hours before the examination, besides using comfortable clothing and shoes for taking the test.
Vital signs were measured before and at the end of the test: -Systolic (SBP) and diastolic blood pressure (DBP), using a Diasist stethoscope and BD sphygmomanometer, through indirect auscultation; -Cardiac frequency (CF) and peripheral oxygen saturation (SpO 2 ), using a Nonin 8500A portable pulse oximeter; -subjective feeling of dyspnea, using Borg's perceived exertion scale, ranging from "zero" for no lack of air to "ten" for a maximum feeling of lack of air.
The six-minute walking test involved a walk during which the patient was asked to walk the longest possible distance, during a six-month period, receiving a standardized encouragement every minute (10) .
Patients were accompanied by the evaluator during the six minutes and continuously monitoring through the pulse oximeter.For the sake of analysis, CF, SpO and were asked to adopt diaphragmatic breathing (11)   during the application.

STATISTICAL ANALYSIS
For the statistical analysis of physiological variables and distance walked before and after treatment with NIV, we used Wilcoxon's nonparametrical test, as data did not present a normal distribution.A p<0.05 significance level was adopted.

DISCUSSION
Noninvasive ventilation has been used in different studies to provide greater respiratory muscle rest (4) .NIV is able to "alleviate" the inspiratory muscles' work load, promoting a temporary rest and thus allowing for better conditions to develop respiratory muscle strength.
Our results showed that NIV through a nasal mask by means of BiPAP ® , during a six-week period, significantly increased muscle strength in COPD patients, in line with literature (12) , which has demonstrated increases in MIP and MEP after the chronic application of BiPAP ® in COPD patients.
Increases in respiratory muscle strength through the use of pressure support have also been observed (13) when applying NIV to COPD patients during the night.
Moreover, our results indicated a probable improvement in exercise tolerance after treatment with NIV, as we found a significant increase in the distance walked during the 6MWT which, although very simple, has been frequently used in field studies (14) .
Despite the physical limitations COPD patients normally present, the distance walked by all of our patients exceeded 54 meters, as shown in Figure 1.
This has been mentioned (15) as a good indicator of these patients' clinical improvement.
Similar results were found in a research (3) that assessed the effects of BiPAP ® when applied for two hours during the day, for one week.This research analyzed severe but stable COPD patients, eight of whom were treated with a placebo method (without NIV) and seven with BiPAP ® two hours per day for five consecutive days.These authors (3) demonstrated that BiPAP ® improved tolerance and reduced patients' dyspnea, while the placebo group did not obtain any significant improvement.However, the study neither evidenced a clinical improvement nor an increase in these patients' respiratory muscle strength (15) .
Noninvasive ventilation, as a resource to improve respiratory muscle strength and physical performance, may require a longer treatment time, involving orientations about diaphragmatic breathing (11) .This is characterized as Functional

2 and
subjective feeling of dyspnea were recorded before and after the test.With a view to minimizing learning effects, each patient carried out two tests before the treatment, and the longest distance was calculated.Noninvasive Ventilation (NIV) Patients were submitted to NIV by means of BiPAP ® , using a nasal mask, for 30 minutes, three times per week, on alternate days, during six weeks.BiPAP ® levels were adjusted according to each patient's tolerance.Patients remained comfortably seated throughout the NIV application (with IPAP set between 10 and 15 cmH 2 O and EPAP at 4 cmH 2 O),

Figure 1 -
Figure 1 -Individual values for distance walked during 6MWT, obtained during evaluation and reevaluation

Table 1
BMI: body mass index; SD: standard deviation.

Table 3
®SpO 2: peripheral oxygen saturation; CF: cardiac frequency; MIP: maximum inspiratory pressure; MEP: maximum expiratory pressure.Figure 1 illustrates individual results for distance walked in meters, obtained before and after treatment with NIV.